Field of the Invention
The present invention relates to a laser beam spot shape detection method for inspecting a spot shape of a laser beam oscillated from laser beam oscillation means of a laser machining apparatus and collected by a condenser.
Description of the Related Art
In a semiconductor device manufacturing process, the surface of an approximately disk-shaped semiconductor wafer is partitioned into a plurality of areas by scheduled division lines that are formed in a grid pattern, with a device such as IC or LSI formed in each of the partitioned areas. A semiconductor wafer formed as described above is cut along scheduled division lines, thus dividing the wafer into areas, each having a device formed therein and manufacturing individual device chips. An optical device wafer having a gallium nitride based compound semiconductor or the like stacked on the surface of a sapphire or silicon carbide substrate is similarly cut along scheduled division lines, thus dividing the wafer into individual optical device chips such as light emitting diodes and laser diodes for application to a wide range of electronic equipment.
In order to divide a wafer along scheduled division lines as described above, a method has been proposed which forms laser-machined grooves, break start points, by irradiating a pulsed laser beam at a wavelength that is absorbed by the wafer along scheduled division lines, applies an external force along the scheduled division lines where the laser-machined grooves as the break start points are formed, and thereby cuts the wafer (refer, for example, to Japanese Patent Laid-Open No. 2006-294674).
Further, other attempts are also being made to divide a wafer along scheduled division lines as described above. Such attempts include a laser machining method that uses a pulsed laser beam at a wavelength that passes through the wafer and irradiates the laser beam with the focal point thereof positioned inside the area of interest. This division method using a laser machining approach is designed to continuously form modified layers inside a wafer along scheduled division lines by irradiating the scheduled division lines with a pulsed laser beam at a wavelength that passes through the wafer from one side of the wafer with the focal point thereof positioned inside the wafer and break and divide the wafer by applying an external force along streets whose strength has declined as a result of formation of the modified layers (refer, for example, to Japanese Patent Laid-Open No. 2006-12902).
However, the condenser adapted to collect a laser beam includes a combination lens made up of a number of convex and concave lenses. Besides, there is a distortion in the optical system from the laser oscillator to the condenser. As a result, a laser beam is not always focused into an intended shape such as circular. If there is a distortion in the shape of a focused laser beam, machining cannot be conducted in a desired manner.
In order to solve such a problem, a spot shape detection method is disclosed in Japanese Patent Laid-Open No. 2013-22634. This method detects a spot shape by holding a detection substrate having a light emitter made of extremely small particles with chuck table, moving the chuck table in the X- and Y-axis directions while at the same time irradiating the light emitter with a laser beam, and detecting the spot shape from the coordinates of reflected light from the light emitter.